Surface-modifying metal articles by action of an impregnating or alloying metal and composition therefor



Patented June 22, 1954 UNITED STATS SURFACE -MODIFYING METAL ARTICLES BYACTION OF AN IMPREGNA'EKNG R ALLOY- ING METAL AND @QMPOSEEIGN THERE- FORWallace C. Johnson, iehairer Heights, Uhio, as-

signor to The Duriron ilompany, line, Dayton, Ohio, a corperation of ewYork No Drawing.

Application August 24, 1950,

Serial No. 181,312

(oi. ll.7--65) 1 21 Claims.

This invention relates to surface-modifying metal articles by action ofan impregnating or alloying metal; and it has to do more particular hrwith a process of surface-modifying metal articles which is essentiallycharacterized by the inclusion of a step or stage wherein the surface ofthe metal article or "Work to be altered, or such portion of it as is tobe impregnated or cased, is exposed, while maintained at a reactiontemperature within a range sufficiently below its melting point toprevent its undergoing either liquation or substantial deformation, tothe action of the impregnating or alloying metal in available formcontained in a layer of sintered (i. e. incipiently or partially fused)but suitably refractory material, resembling in its generalcharacteristics a ceramic or slag-like composi tion which, at the statedtemperature, has a pasty consistency and adheres to the surface of saidarticle, thereby shielding it from the atmosphere of the heating furnacewhile maintaining the source of the impregnating or alloying metal inintimate contact with said work.

Surface modification of metal articles for various purposes, andparticularly such treatment of ferrous metal articles, in order torender them more resistant to rusting and other types of corrosion, aswell as to enhance their ability to withstand mechanical wear,constitutes the subject matter of a very extensive prior patent andjournal literature. Some of the methods proposed. have found more orless successful practical ap plication in industry. Generally speaking,however, commercial success has been far from outstanding where thesurface-modifying procedure has involved impregnating orsurface-alloying an iron or steel article with such a metal as silicon,for example, to provide the article with a corro-- sion and wearresistant case. All the methods of achieving this that have beenproposed. heretofore, insofar as they are capable of producing areasonably satisfactory case, are cumbersome, more or less complicated,and generally not commercially practicable. Some involve heating thearticles in special mixtures of granular solid materials. Others requirethe use of fused salt baths, or of special atmospheres, air tightcontainers or reaction chambers. Long heating cycles at very hightemperatures are the rule, with close control of operating conditions atall stages usually desirable but difficult to attain. All these factorshave greatly restricted application of such processes, even where theirrela tively extremely high cost-might not in itself be prohibitive.

It is an object of the present invention to provide a greatly simplifiedand relatively inexpensive procedure whereby ferrous and other metalarticles may be materially improved as regards resistance to corrosionespecially, and commonly to wear also, through impregnation orsurfacealloyin with a metal or a plurality of metals which, singly ortogether, are capable of conferring upon the base metal (1. e. the metalof which the given article is composed.) the improved property orproperties sought.

A further object of the invention is to extend materially the practicalapplication of metal impregnation and surface-alloying for theprotection of ferrous and other metal articles.

With the foregoing general objects in view, as well as others which willbecome evident hereinafter, the nature of the invention will now be morefully set forth and explained by describing in detail certain specificembodiments of the invention that are regarded as particularlyadvantageous and will serve to make clear the underlying principlesinvolved. These embodiments should be understood, however, to be onlyillustrative of many specifically different forms which the novelprocess may take within the scope of the invention pointed out in theaccompanying claims.

In general outline, the process of the invention may be described as onein which the metal article whose surface is to be altered by theimpregnating metal is provided initially with a substantially dry,adherent coating or enclosing layer of a mixture comprising finelydivided refractory slag-forming materials and a temporary binder torender the mixture coherent, said mixture including the impregnatingmetal either as such or in other available form, and one or more c e sadapted. to facilitate sintering (i. e.

pregnating metal upon the underlying or basis metal surface; the coatedarticle is then heated to a temperature that is high enough to sinter orpartially fuse the adherent mixture into a rather viscous agglomeratewhich clings to the surface of said article and substantially seals thesame, said temperature being insufficiently high, however, to render themixture thinly fiuid or endanger the integrity of said article; theheating is continued within this temperature range until surfacemodification of the metal article by the impregnating or alloying metalhas been accomplished to the extent desired; the resultant product isthen cooled to solidify the adherent sintered slag-like coating, whichis desirably more or less glassy or porcelain-like in character; andsuch of the solidified coating as may still adhere to the cooled articleis usually removed.

together with the impregnating metal or source compound thereof,preparatory to furnacing the thus-coated article, is one which althoughsimple enough from the standpoint of manipulative procedure, must becarried out with proper care and attention to certain operationaldetails necessary to observe in order to obtain optimum results. This isbecause the properties of the protective case formed by the action ofthe impregnating metal in the ensuing furnacing or firing operationdepend in large part upon the characteristics, both physical andcompositional, of that initial coating layer which, for convenience, isherein referred to as the preliminary (i. e. green or unfired) ceramicblanket or envelope.

This preliminary blanket or envelope is formed by suitably applying tothe metal article, as by dipping, brushing or spraying, a fluid,slurrylike composition generally similar in its characteristics to a wetgrog or slip, and therefore commonly so termed herein. Its exactcomposition may vary Widely within the scope of the invention, dependingupon the nature of the metal or alloy of which the article is composed,and the nature of the impregnating metal, factors which will necessarilydetermine the most suitable furnacing or firing temperature to beemployed in any given instance, and hence what should be the compositionof the slagor glass-forming mixture in order for it to have the desiredviscous consistency at such furnacing temperature. Essentially, however,the grog or slip comprises a suspension, in a reasonably volatile liquidvehicle that may in some cases be aqueous and, in

others, more desirably non-aqueous, of the various finely divided solidmineral materials including the impregnating metal source, i. e. eitherthe metal as such or in the form of a compound from which it becomesavailable in the firing operation, which are to be present in thepreliminary ceramic blanket or envelope aforesaid; plus a binder whichholds said materials firmly coherent in the. blanket or envelope duringand after evaporation of the liquid vehicle therefrom prior to firing.Said binder may desirably be a substance that is soluble in saidvehicle.

As already indicated, the composition of the grog should be such as toprovide a more or less vitreous slag that will serve as a carrier mediumfor maintaining the available impregnating metal in very intimateshielded contact with the surface of the metal article during the firingor furnacing operation; and to this end the grog composition shouldalways include ingredients adapted to produce such a slag-like carrier.The slag-forming mixture should have its mineral components soproportioned that its sintering or incipient-fusion temperature ortemperature range reasonably approximates the firing or furnacingtemperature that is to be employed in said firing operation. This isreadily done by applying compositional principles well known in theceramic art. As a general rule it is highly desirable for achievement ofoptimum results, although not invariably indispensable, that theingredients of the grog employed in practicing the invention shallinclude a halogen-containing, more particularly a fluorine-containing,fiuxing agent such, for example, as a halide of an alkali metal oralkaline earth metal, or a complex halide of more than one such metal.

At the present time, the invention finds its most important practicalapplication in the impregnation, with silicon, of ferrous metalarticles, more especially steel and fabricated steel prodthat of theunderlying metallic case.

nets of all kinds, to form thereon'ajcorrosion resistant layer or case.This particular aspect or phase of the invention will accordingly bemore especially referred to and emphasized in the following detaileddescription notwithstanding the iact that, in general, the invention isapplicable to the impregnation or surface-alloying of any metal articleor product with any other metal capable of penetrating thereinto at atemperature below the melting point of the basis metal.

Let it be assumed by way of example that the process is to be applied toa plate, tube or bar of mild steel. The procedure in this case maydesirably consist of the following steps:

(a) Sand-blasting or otherwise cleaning the steel articles to betreated.

(b) Coating the articles by dipping them in a slip or slurry-like grogof suitable fluidity, consisting of ferrosilicon, sodium fluoride andsilica, together wtih a liquid binder.

(c) Drying the coated articles, prior to firing, to drive off thevolatile portion of the liquid binder. Where the grog coating isrelatively very thick, the drying operation may desirably be continuedat increasingly high temperatures into what amounts to a pre-firingoperation (e. g. at 1200 F.), since this tends to preserve the integrityof the grog coating and its good adherence to the work throughout theremainder of the process.

((1) Firing the dry (and, if necessary, prefired) grog-coated articlesin a suitable furnace chamber at a temperature within the range of 1800to 2000 F., say approximately 1950 F. in a typical instance, to effectimpregnation of the surface of the articles with silicon; the grogcoating or envelope being converted into a viscous slag which serveseffectively to protect the underlying metal surface and thus obviate thenecessity of employing any special furnace atmosphere. A good way tosupport the articles in the firing furnace is to place them upon a layerof silica flour, for example.

(6) Cooling the fired articles to ordinary tem perature, and removing,as by hammering or tumbling, any of the vitrified slag coating which maynot have spalled off during the later stages of the cooling. It isindeed desirable to so proportion the ingredients of the grogcomposition as to encourage this spalling off of the slag coating orenvelope in the later stages of the cooling down. This is merely amatter of adjusting the composition of the vitreous material so that itscoeflicient of expansion differs sufiiciently from spalling off of thefired ceramic blanket may also be encouraged by water quenching the workfrom. a red heat.

The solid components of the fiuid grog should all be in powdered orfinely divided form. In preparing the grog, the solid ingredients arecommonly dry-mixed as powders and then suspended in the liquid bindercomposition by adequate stirring or other mode of agitation.

Ferrosilicon is ordinarily used as the source of silicon because of itsready availability and relatively low cost. It should be used in theform of a powder which will all pass through a 100- mesh sieve, withapproximately passing through ZOO-mesh. The silicon content should benot less than 20% by weight and may run as high as It is possible to useeven silicon metal in powdered form, but the cost is much greater and itappears to offer no advantage over ferrosilicon for the purposes of thisinvention. As a general rule, use of a ferrosilicon containing from 39%to 75% silicon gives the most satisfactory results in practice. It issometimes found desirable to use in the same grog ferrosilicons ofdifferent silicon contents in order to obtain satisfactory suspension ofthe particles of solid matter. A ferrosilicon of high silicon contentis, of course, of lower density than one of lower silicon content.Furthermore, by using ferrosilicons of differing silicon contents insuitably proportioned blends or mixtures, it is possible to obtain slagblankets that spall off automatically or, at any rate, are ratherremovable, upon cooling the fired article.

It is possible, of course, to employ available silicon in forms otherthan those mentioned here inabove. Thus, silicon carbide could in somecir cumstances serve as the silicon source; the considerably higherfiring temperature which its use requires is a factor sufficient initself, ordinarily, to render the use of ferrosilicon, for e:-:- ample,decidedly more advantageous.

As regards the fluoride component of the grog or slip, sodium fluoride,in the form of a dense white powder of such fineness that it will allpass ZGil-mesh, has been found usually to give optimum results. Asidefrom the fact that it acts as a flux, the exact function of the fluorideis not fully understood. However, it is believed possible that, at thetemperature at which silicon impregnation of the underlying steel iseffected in the furnacing operation, the fluoride may serve as a sourceof fluorine available to promote, perhaps catalytically, the penetrationof silicon into the underlying steel surface. From this standpoint, the

fluoride may be regarded as a .ixing catalyst; but, regardless oftheory, it obviously has a most favorable effect upon the impregnatingor alloying action or reaction, expediting it materially. Otherfluorine-bearing compounds can also be used for this purpose instead ofor in conjunction with sodium fluoride. Among such compounds are, forexample, barium fluoride, calcium fluoride, sodium fluosilicate (orsilicofluoride) also the corresponding potassium salts. Chlorides oi thealkali metals and alkaline earth metals, as well as of other metals, mayalso sometimes be usefully included as fluxing agents, but in generalare not as effective as fluorides in promoting the penetration of theimpregnating or alloying metal into the work being treated.

The silica may be in the form of silica flour. However it is oftenadvantageous to employ, instead, silica aerogel because of its lowerapparent density and consequent greater eifectiveness as a suspensionagent for the fluoride and ferrosilicon. A silica aerogel available fromMonsanto Chemical Company under the trade name Santocei is satisfactory.The air volume within the silica particle is about 94% 'of thisparticular form of silica aerogel, which therefore has great powers ofsuspension. Besides acting as a suspending agent, this or any other formof silica employed in the grog supplies at least a part of the acidcomponent required for fluxing the basic constituents of the grog.

The grog composition can be rendered especially eifective if itcontains, in addition to the ingredients already mentioned, a soda limeor .borosilicate glass in powdered form all passing through ZGO-mesh.This is of somewhat lower melting point than the vitreous slag or glassymaterial that would be formed Without this addition. Moreover, thisaddition provides a con venient means of controlling and regulating thefluidity of the vitreous blanket produced in the firing operation sothat it shall be of the desired degree of viscosity. This viscosityshould be such that the blanket or envelope does not flow materially atthe maximum firing temperature employed, but yet is soft enough topermit escape of occasional gas bubbles outwardly there-through, withprompt rescaling of the vent openings to restore substantial gastightness of the envelope.

The liquid binder employed may vary widely in character, and theviscosity of the grog, which is of great importance in obtaining properapplication of the grog to the work initially, is of course controlledby the amount of liquid binder that is mixed with the powdered solidcomponents. The liquid binder is a solution or dispersion of anagglutinant binding material proper in a solvent or liquid vehicle.While the active binder component may be either organic or inorganic,the best results are ordinarily obtained by using an organic material,such as a cellulose ester, for example, carried by an appropriatenon-aqueous solvent vehicle. For example, a satisfactory liquid bindercomposition for present purposes may be obtained by diluting one part byvolume of an ethyl-cellulose clear lacquer (e. g. Du Pont #1234) withthree parts of a fast drying lacquer thinner (e. g. Du Pont #3651). Ingeneral, the liquid binder employed should be such that, when a grogemploying it is applied to the work and the Work is charged directlyinto a hot furnace after a preliminary heating-up to drive oif thevolatile solvent, the coating even when relatively thick will not flakeor blister. In actual practice, of course, the work, after being coatedwith the grog, is heated moderately to drive oif the solvent and dry thegrog-coated work before firing.

Although the use of water or a partly aqueous medium as the liquidportion of the liquid binder composition is frequently quiteimpracticable be cause of the very slow rate at which the grogcoatedwork would have to be dried in order to avoid causing the coating toflake or blister, and because of the difficulty of eliminatingsubstantially all traces of the liquid vehicle in the drying operation,nevertheless water may be employed to advantage where such slow rate ofdrying is not seriously objectionable, because of the sub stantialsaving in cost of liquid vehicle thereby realized. In such case, it isfeasible to use sodium silicate or a water-soluble gum, for example, asthe active binder component.

The desired case thickness is obtainable by heating the work long enoughat the temperature most practical under the circumstances. The higherthe temperature and the longer the time of heating, the thicker is thecase. For cases of high silicon iron the temperature should not exceed2100 F. At about this temperature, the sintered coating or envelope,which up to this point remains properly viscous or plastic, starts tobecome too fluid and, at 2200 FL, will run oif the work. Thin cases ofsilicon iron will form as low as 1550 F. Generally speaking, operationof the present process at much below about l'iili) F. is notcommercially practicable. The most desirable practical operating rangeseems to be from about 1750 to 1959 or 2000" F., with 1950 F.representing typical good practice.

Thin cases of high silicon iron (1--l l% sill con) will form in threeminutes at l850 Operating on -inch steel plate at a furnace temper'atureof 1950 F., high silicon iron cases of varying thicknesses dependingupon the length of firing time are obtainable as follows:

At heat: Case thickness minutes 0.007 inch. minutes 0.017 inch. minutes0.031 inch. 90 minutes 0.047 inch.

For steel parts that require machining after the protective case hasbeen formed, it is usually necessary to hold the furnace temperature fortwo to three hours in order to form a case that will provide sufficientthickness for machining.

The heat used to produce the reaction that forms the protective case canbe derived from gas, coal or oil. Radiant heat from electric furnaces orinduction heating can also be used. No special furnace atmosphere isrequired.

In order to further illustrate the principles of the invention asapplied to siliconizing metal articles or products, more particularlythose composed of steel, several specific examples will now be given. Inthe various formulae, the proportions are given in parts by volumeunless other wise indicated.

Example I In this instance the grog or slip has the followingcomposition:

1 part silica flour 2 parts sodium fluoride (dense white grade)- throughZGO-mesh 6 parts 50% ferrosiliconthrough 150-mesh 6 parts 90%ferrosilicon-through 150-mesh These ingredients are mixed dry, and thedry mixture worked into a dipping slurry by the use of a liquid bindercomposition in the form of an ethyl-cellulose clear lacquer. The work iscoated by dipping it in this slurry or grog while keeping it constantlystirred to prevent settling. After thoroughly drying the coat by gentlyheating the work to evaporate off the volatile portion of the liquidbinder composition, firing the work at a temperature of 1950 R, usuallyfor as little as 15 minutes, gives an excellent case approximately 0.015inch thick.

By substituting silica aerogel (e. g. Santocel) partly or wholly, forsilica fiour as a suspending agent, the tendency of the other solidcomponents of the grog to settle out is greatly lessened. Also itbecomes possible to apply a heavier coating of grog to the work withoutencountering so much tendency of the grog to slip from the work beingtreated. Where such heavier coating of grog is applied, it is generallywell to slow down the drying time of the liquid binder compositionsomewhat by adding thereto a suitable proportion of a slower dryingthinner or solvent, or using less thinner, in the manner well known inthe lacquer and paint arts. Where the binder composition dries too fastthere is a tendency on the part of the green ceramic blanket or envelopeto crack and blister, resulting in an imperfect case upon firing. It isvery important that the dried grog coating be as uniform and smooth aspossible, and that it adhere firmly to the surface of the underlyingwork at all points.

Example II In this procedure, which is especially suitable where theproduction of a relatively thick (e. g. 0.05 inch or more) high siliconiron case on the work is desired, the grog should be applied to the workin several successive coats, each being dried before application of thenext coat.

8 In this instance, the first coat is a thin priming coat of grog havingthe following formula as regards its solid components, which are finelypowdered as before:

1 part silica aerogel 2 parts sodium fluoride 12 parts ferrosilicon Thispriming grog provides thorough and immediate coverage of the work. Theearly thin case thus gives protection from oxidation during prolongedfiring periods.

For the second and subsequent coats, the grog formula as regards solidcomponents may be as follows:

6 parts silica aerogel 3 parts sodium fluoride 3 parts AFSL (air floatedsoda lime) powdered glass 12 parts 50% ferrosilicon The purpose ofincluding the air floated soda lime glass in this grog formula is tofavor early interaction of the grog ingredients in the firing. Its

ction is to cause early fluxing of the acid and basic elements of thegrog.

Example lll When iron oxide in powdered form (all through ZOO-mesh) isadded to any of the formulae given in the foregoing examples, inproportion up to 50 of the combined volumes of the ferrosilicon andsodium fluoride, such addition promotes an exothermic reaction as thesilicon case forms at elevated temperatures, and this may increase tosome extent the efiectiveness of the action of the iiuxing catalyst,thereby permitting the use of lower furnace temperatures.

Example IV By adding certain other metals in available form to the grogor slip, such metals may also be caused to penetrate the underlyingmetal surface along with silicon, thus producing cases of modifiedcharacteristics. Thus, in any or" the foregoing examples less brittlehigh silicon iron cases can be formed by adding copper to the indicatedsiliconizing formula in volume proportion of about 1 part of fine copperpowder (through 300-mesh) to 12 parts of ferrosilicon. This, when firedin the usual way, forms a low copper silicon alloy that is somewhat moreductile but has lower corrosion resistance. An even better result isobtainable by adding the copper in the form oi red copper oxide.

Additions of manganese (e. g. as ferromanganose) to the ferrosilicontend to make a harder case and also to give the case surface a silverybrightness. Also, adding manganese causes somewhat greater fluidity bothof the case and of the ceramic envelope in the firing operation, thustending toward formation of a more eutectic alloy.

Adding powdered nickel to the ferrosilicon in proportion of from 1 to 2parts of nickel to 12 parts of ferrosilicon has a pronounced effect onthe case, forming what appears to be a finer grain structure which,under prolonged oxidation tests, gives better protection to the steelbase than do silicon cases containing no nickel. Here again, the nickelapparently forms a eutectic alloy which enables the protective case toform at lower temperatures.

Ordinary furnace atmospheres during the firing would have definitelyadverse effect upon the characteristics of the silicon-iron case formed,as by causing formation of a loose superficial case, for example, it mayoften be desirable to further guard against this possibility, withoutresorting to prepared. atmosphere protection. This can be done byfinally dipping the work, to which a grog coating has already beenapplied as in any of the foregoing examples, in a special grog or sliphaving the following volume formula as regards its solid components:

1 part iron oxide (F6203) 2 parts borosilicate powdered glass This givesan especially effective sealing effect during the firing.

In the various formulae given in the foregoing examples, the proportionsof the various ingredients may vary considerably without greatlyaffecting the characteristics of the case obtained, provided care betaken to observe the requirement that, in the furnacing operation, theresultant slag that is formed shall not become completely molten orsubstantially fluid at the firing temperatures employed. However, theformulae given represent good practice, and reasonable approximation tothem is therefore recommended for best results.

Where the invention is to be applied to chromizing a metal article, theprocedure is similar to that hereinabove described for siliconizing, butwith available chromium replacing available silicon in the grog or slipapplied to the work, and

the firing temperature being in general somewhat higher. To provideavailable chromium, ferroohrome in powder form (all through lOO-mesh)may be used, taking the place of ferrosiiicon in the siliconsizingformulae given hereinabove. The carbon content of the ferrochrome shouldmost desirably not exceed about 0.10. High carbon ferroehrome will notform a satisfactory chrome-iron case.

Good chrome-iron cases that can be bent without rupturing the case canbe produced by using low carbon ferrochrome powder (through 100- mesh)in a grog or slip of the following formula:

2 parts silica aerogel 2 parts sodium silicofluoride 6 parts ferrochromemixed to the consistency of paint with a suitable liquid bindercomposition such as a thinned ethyl cellulose lacquer.

The invention can also be applied to formation of a bright metalliccopper case over a steel base. To this end, metallic copper or, stillbetter, red copper oxide, both in powder form (all through loll-mesh),may replace ferrosilicon in the foregoing siliconizing grog formulae.

A further application of the invention is in forming a protectivesilicon case on solid molybdenum and/or titanium metal which, whenunprotected, is very easily oxidized at elevated temperatures.

The adherence of porcelain enamel to a steel base may be substantiallyincreased byflrst providing the steel base with a case of silicon,chrome or oth r suitable metal that will alloy with such base in themanner herein disclosed, thus lessen-- ing the well-known tendency ofporcelain to chip or flake from its base.

In the best mode of practicing the invention, the green but dry grog orceramic coating applied to the work preparatory to firing usually has athickness on the order of not much more than approximately four timesthat of the metallic case that subsequently forms underneath it. Wherethe coatings are relatively thick, say on the order of one-fourth inchor more, there is greater danger than with thinner coatings that themetal-to-coating contact may be disturbed in the subsequent manipulationof the coated work. Thus, in order to minimize cracking and separationof such thick grog coatings from the underlying metal due to thermalshock, it is advisable not merely to dry the work at temperaturesmoderately above the vaporizing point of the liquid vehicle employed inthe fluid grog or slip applied, but also to pro-fire the coated work ata temperature on the order of 1200 F. or thereabouts, before introducingit into the firing or reaction furnace, wherein the temperature iscommonly maintained at from 1800 to 2000 F.

The novel mixtures of finely divided solid materials herein disclosed asuseful in practicing the novel process above described, whether saidmixtures or compositions be in dry pulverulent form or worked up into awet slurry or slip with a suitably volatile liquid vehicle, are to beunderstood as constituting a part of the invention. When in dry form,prior to being worked into a slurry or slip for application to the workto be treated, such a mixture may or may not include, alreadyincorporated therein, a solid binding or cementing agent which issoluble in or miscible: with the subsequently added liquid vehicle insuch manner as to exert the desired binding action in the dried butgreen coating of raw mix with which the work is provided prior tofiring, and also to aid in promoting good adherence of said coating tothe worl; prior to the sintering stage. Thus, where a non-aqueous liquidvehicle is to be employed, the dry mix may have ethyl cellulose, avarnish gum or resin, a solid bitumen, or the like, or any two or moreof these, incorporated therewith in powder form to provide the bindercomponent; said liquid vehicle being so selected or composed as to havethe proper solvent or disper-sing action thereupon when eventuallyemployed. If, on the other hand, an aqueous liquid vehicle is to beemployed in preparing the wet grog or slip, the binder component thusoptionally pre-incorporated in the dry mix may be any one or more ofmany substances available in powder or other suitably comminuted formwhich, upon solution in or mixture with water, exerts effectiveagglutinating action upon the other mix components. Examples of suchsubstances are sodium silicate, starch, a starch derivative orconversion product such as dextrin, or animal lue.

What is claimed is:

1. The process of surface-modifying a metal article by the action of animpregnating or alloying metal, which comprises providing the articleinitially with a substantially dry, adherent coating comprising anunsintercd mixture of finely divided slag-forming solid mineralmaterials associated with an agglutinant binder that renders the mixturecoherent, said mixture including a source of the impregnating metal,silica and a metal halide, so proportioned as to facilitate sintering ofthe mixture at suitably elevated temperature nondestructive to saidarticle and promote action of the impregnating metal upon the underlyingor basis metal surface; then heating the coated article to a temperaturenot lower than about i550 F. that is high enough to sinter or partiallyfuse the coating mixture to viscous consistency and therebysubstantially seal the underlying surface of said article, but isinsufficiently high to render the coating mixture thinly fluid or toendanger the integrity of said article; continuing the heat ing withinsuch temperature range until the sired impregnation has been effected;then cooling the article and removing therefrom sin tered coatingmixture adhering thereto.

2. The process set forth in claim 1, wherein the coating with which themetal article is initially provided is formed by applying to its surfacea fluid grog comprising said mixture and a liquid vehicle in which saidbinder is dispersed and which is readily volatilizable, and said liquidvehicle is evaporated off to dry the grog coating prior to the sinteringoperation.

3. The process set forth in claim 1, wherein at least a part of themetal halide component is a fluoride.

4. The process set forth in claim 3, wherein said binder is an organicsubstance, and said liquid vehicle is non-aqueous.

5. The process set forth in claim 2, wherein said grog coating is builtup on the work to a desired final thickness by applying the fluid grogin a plurality of successive steps and drying the coated work after eachapplication.

6. The process set forth in claim 2, wherein said grog coating is builtup on the work to a desired final thickness by applying the fiuid grogin a plurality of successive steps and drying the coated work after eachapplication, the grog appl ed in the first step being thinner than thatsubsequently applied.

7. The process set forth in claim 2, wherein said grog coating is builtup on the work to a desired final thickness by applying the fiuid grogin a plurality of successive steps and drying the coated work after eachapplication, the ap plied in the first step being thinner than thatsubsequently applied, and the composition of the subsequently appliedgrog being such as to favor fiuxing interaction of the grog ingredientsat lower firing temperature than that of the grog first applied.

8. The process of surface-modifying a metal article by impregnating itwith silicon to form a corrosion-resistant case thereon which comprisescoating such metal article with a fluid grog or slip comprising amixture of finely divided solid mineral materials including a source ofsilicon, silica and a metal halide fiuxing agent, so proportioned as torender the mixture sinterable at a temperature non-destructive to saidmetal article, associated with, an agglutinant binder and a readilyvolatilizable liquid vehicle in which said binder is dispersed;evaporating off said liquid vehicle to leave a substantially dry,coherent coating adhering to the article; and firing the article at atemperature not lower than about 1550 F. that is sufiiciently high tosinter the coating mixture to viscous consistency, but not high enoughto render it thinly fluid or to deform the article; finally cooling thearticle and removing any sintered material adhering thereto.

9. The process set forth in claim 8, wherein the metal of the article issteel, and said grog comprises essentially a mixture of ferrosilicon,silica and sodium fluoride, associated with said binder and a sufficientquantity of said liquid vehicle to give the mixture a slurry-likeconsistency.

10. The process set forth in claim 9, wherein the silica content of thegrog is at least partly in the form of an aerogel.

11. The process set forth in claim 10, wherein the ferrosilicon employedin the fluid grog comprises different grades as regards silicon content,but the overall ferrosilicon component analyzes, by weight, not lessthan 20 per cent silicon.

12. The process set forth in claim 9, wherein the grog mixture furthercontains powdered iron oxide in proportion up to 50 per cent by volumeof the ferrosilicon and sodium fluoride contents taken together.

13. As a new composition of matter, useful in surface-modifying metalarticles by the action of an impregnating or alloying metal, a mixturecomprising a source of such impregnating metal, together with silicaand. a metal halide fluxing component, all intimately commixed in finelydivided condition and so proportioned that the mixture will sinter orpartially fuse into a viscous slag-like mass at a predeterminedtemperature within the range of 1500 to 2100 F.

14. The new composition of matter defined in claim 13, wherein the metalhalide component includes a fluoride.

15. The new composition of matter defined in claim 14, wherein siliconis the impregnating metal.

16. The new composition of matter defined in claim 13, wherein at leastpart of the silica content is in the form of an aerogel.

17. The new composition of matter defined in claim 13, wherein siliconis the impregnating metal and at least part of the silica content is inthe form of an aerogel.

1s. The new composition defined in claim 13, said composition being insubstantially dry, noncohering condition, and further including a finelydivided solid agglutinant binder component dispersible in a given liquidvehicle with which the mixture is to be used to produce a slurry-likecomposition.

19. The new composition defined in claim 13, wherein at least part ofthe silica content is in the form of an aerogel, said composition beingin substantially dry, non-cohering condition and further including afinely divided solid agglutinant binder component dispersible in a givenliquid vehicle with which the mixture is to be used to produce aslurry-like composition.

20. The new composition defined in claim 13, mixed with a liquid vehiclein proportion to produce a mass of slurry-like consistency, said massincluding an agglutinant binder dispersed in said liquid and effective,upon evaporation of said vehicle, to render the residual mass coherent.

21. The new composition defined in claim 13, wherein at least part ofthe silica content is in the form of an aerogel, said composition beingmixed with a liquid vehicle in proportion to produce a mass ofslurry-like consistency, said mass including an agglutinant binderdispersed in said liquid and effective, upon evaporation of saidvehicle, to render the residual mass coherent.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date Re. 20,719 Ihrig May 10, 1938 928,398 Patterson July 20, 19091,735,000 Dely Nov. 12, 1929 1,784,570 Becket Dec. 9, 1930 1,853,370Marshall Apr. 12, 1932 2,109,485 Ihrig Mar. 1, 1938

1. THE PROCESS OF SURFACE-MODIFYING A METAL ARTICLE BY THE ACTION OF AN IMPREGNATING OR ALLOYING METAL, WHICH COMPRISES PROVIDING THE ARTICLE INITIALLY WITH A SUBSTANTIALLY DRY, ADHERENT COATING COMPRISING AN UNSINTERED MIXTURE OF FINELY DIVIDED SLAG-FORMING SOLID MINERAL MATERIALS ASSOCIATED WITH AN AGGLUTINANT BINDER THAT RENDERS THE MIXTURE COHERENT, SAID MIXTURE INCLUDING A SOURCE OF THE IMPREGNATING METAL, SILICA AND A METAL HALIDE, SO PROPORTIONED AS TO FACILITATE SINTERING OF THE MIXTURE AT SUITABLY ELEVATED TEMPERATURE NONDESTRUCTIVE TO SAID ARTICLE AND PROMOTE ACTION OF THE IMPREGNATING METAL UPON THE UNDERLYING OR BASIS METAL SURFACE; THEN HEATING THE COATED ARTICLE TO A TEMPERATURE NOT LOWER THAN ABOUT 1550* F. THAT IS HIGH ENOUGH TO SINTER OR PARTIALLY FUSE THE COATING MIXTURE TO VISCOUS CONSISTENCY AND THEREBY SUBSTANTIALLY SEAL THE UNDERLYING SURFACE OF SAID ARTICLE, BUT IS INSUFFICIENTLY HIGH TO RENDER THE COATING MIXTURE THINLY FLUID OR TO ENDANGER THE INTEGRITY OF SAID ARTICLE; CONTINUING THE HEATING WITHIN SUCH TEMPERATURE RANGE UNTIL THE DESIRED IMPREGNATION HAS BEEN EFFECTED; THEN COOLING THE ARTICLE AND REMOVING THEREFROM ANY SINTERED COATING MIXTURE ADHERING THERETO. 